Charge rollers minimize the creation of ozone since they contact the surface being charged. Charge rollers normally have an outer layer different from the body of the roller to concentrate charge at the outer layer, to protect the body from deterioration during use, and, if necessary, to prevent chemical interaction between the body of the charge roller and the photosensitive surface being charged.
A typical coating of such existing charge rollers is nylon, application of which is a costly step which employs solvents which must be contained so as not to enter the environment.
Known prior art charge rollers, such as those described in U.S. Pat. No. 5,637,391 to Claflin, assigned to the same assignee to which this application is assigned, employs an economical layer of powder on the outer surface of the body or a charge roller. This is applied by simply contacting the roller with ample amount of the powder and cleaning away the excess, all at much less cost than normal dip coating or the like.
As the speed of printing operation is increased, such acrylic coated charge rollers, when baked, do not sustain sufficient late-life charging (300,000 prints). One reason is that increased speed of the machine results in decrease in contact time between the charge roller and the photoconductive drum. Another reason is contamination, which impairs charging, increases on the charge roller over life. Also, the combination of a new baked acrylic coated charge roller with lower-melting temperature toner used in higher speed printers can result in slight over charging at the beginning of life of the charge roller. This results in unacceptable early-life background levels in the printed images.
In accordance with the observations leading to this invention, it was discovered that a baking process used to secure the acrylic powder caused severe decreases in the effective life of the charge roller when used in a higher speed printer. No baking is an alternative disclosed in the foregoing U.S. Pat. No. 5,637,391. By not baking the acrylic powder onto the charge roller, the life of the charge roller is increased from about 100,000-120,000 prints to around 300,000 prints when used in a combination with a charge roller wiper. This effectively solves the print quality problems associated with late-life charge rollers. Although charge rollers produced this way (unbaked) are capable of maintaining adequate charge levels (greater than 880V) throughout life, the early-life over-charging problem remains. Also, the unbaked charge roll is more sensitive to the effects of handling, which results in increased print defects resulting from disturbances in the powder coating.